The present invention relates to a thin film transistor which is suitable for use, for instance, as an active element of a drive circuit for a liquid crystal display cell, and more particularly to a thin film transistor with an amorphous semiconductor layer which is preferable for use as an active element of a drive circuit for selective switching control of display electrodes of the liquid crystal cell.
Heretofore there has been proposed an active matrix liquid crystal display of the type that has display electrodes arranged in a matrix form and selectively applies thereto voltage to reproduce and display television signals as images. In this instance, thin film transistors are employed for the selective voltage application to the display electrodes of the liquid crystal display cell and drive circuits for driving gate buses and source buses connected to gates and sources of the thin film transistors, respectively, are provided on one of transparent substrates of the liquid crystal display cell. Such an arrangement is proposed in U.S. patent application Ser. No. 510,481 "Dot-Matrix Liquid Crystal Display" filed on July 1, 1983 corresponding to Japanese Patent Application No. 120,808/82 (Pat. Pub. Disc. No. 10,988/84), "Liquid Crystal Color Display", for reducing the number of external leads (terminals) and for facilitating the connection of the liquid crystal display cell with an external device.
A brief description will be given first of this conventional liquid crystal display. As shown in FIG. 1, liquid crystal is sealed in the space defined by transparent substrates 11 and 12 to form a liquid crystal cell 13. In the liquid crystal cell 13 a number of display electrodes and thin film transistors connected thereto via drain electrodes, though not shown, are disposed, for example, in a matrix form on the transparent substrate 11. For each row of the thin film transistors their gate electrodes are connected to a gate bus 14 and for each column their source electrodes are connected to a source bus 15. The gate buses 14 and the source buses 15 are led out of the liquid crystal cell 13. The transparent substrate 11 has formed thereon a gate bus drive circuit 16 for selectively driving the gate buses 14 and a latch circuit (a source bus drive circuit) 17 which is connected to a shift register 18. Picture signals to be displayed are applied serially to the shift register 18 and latched in the latch circuit 17 in parallel. Parallel outputs of the latch circuit 17 are respectively provided on the corresponding source buses 15.
The provision of the gate drive circuit 16 and the latch circuit (the source drive circuit) 17 on the transparent substrate 11, for driving the thin film transistors in the liquid crystal cell 13, permits the reduction of the number of terminals for external connection, and hence is very convenient.
Conventionally the gate drive circuit 16, the latch circuit (the source drive circuit) 17 and the shift register 18 are each formed using an MOSFET, i.e. an FET which has its gate electrode formed on a semiconductor layer between the source and drain through a gate insulating film. On account of this gate insulating film, the MOSFET is low in the operating speed as a switching element, calls for a relatively large drive current and is somewhat complex in structure, and hence is low in production yield and not suitable for fabrication with high integration density. Furthermore, the voltage to be applied is relatively high, that is, since the gate voltage is applied via the gate insulation film to the semiconductor layer, the voltage is divided by the gate insulating film and the voltage applied to the semiconductor layer becomes low, so that an appreciably high gate voltage is needed.
A metal-epitaxial-semiconductor thin film transistor (MESFET) of the type that has a GaAs epitaxial growth layer formed on a semi-insulating GaAs substrate and a metallic layer formed on the epitaxial layer in Schottky contact therewith and controls source-to-drain current by controlling a depletion layer is proposed, for instance, in Proc. IEEE, 54,307, 1966, C. A. Mead, "Schottky Barrier Gate Field-Effect Transistor" and Proc. IEEE, 55, 1237, 1967, W. W. Heuper et al., "An Epitaxial GaAs Field-Effect Transistor". Since the MESFET employs the epitaxial growth layer, it is difficult to use for a circuit which needs a large area, and this FET is expensive because of using GaAs.